In many cases, parts such as gears, pulleys, and shafts for motor vehicles and industrial machinery are manufactured by roughly shaping them by hot forging or cold forging, by subjecting them to machining and thereafter to casehardening by carburizing quenching or carbonitriding quenching. However, at the said casehardening treatment, if austenite grains before quenching are coarsened, there easily arise problems that the fatigue strength as a part decreases and that the amount of distortion at the quenching time increases.
Generally, it has been thought that, as compared with cold forged parts, in hot forged parts, the austenite grains are less liable to be coarsened at the time of carburizing or carbonitriding.
In recent years, however, with the progress of hot forging technique, hot forging has frequently been carried out in various temperature ranges: and thus the number of hot forged parts with the austenite grains coarsened at the time of carburizing or carbonitriding has increased.
Therefore, there has been demanded a hot rolled steel bar or wire rod in which the austenite grain coarsening can be stably prevented at the heating time in the process of carburizing or carbonitriding even if hot forging is carried out in various temperature ranges, and techniques concerning steels and producing methods therefore have been proposed in the Patent Literatures 1 to 3.
To be concrete, the Patent Literature 1 discloses “a grain stabilized carburizing steel” characterized in that the steel with limited amounts of sol.Al and N and a limited ratio of “sol.Al/N” is heated to a temperature of 1200° C. or more and thereafter is hot worked.
The Patent Literature 2 discloses “a steel for high temperature carburizing having an excellent property in high temperature carburizing, and a hot forged member for high temperature carburizing” characterized in that the elements such as Al, Nb and N are contained in a specific amount respectively, and the precipitation amounts of Nb(C, N) and AlN after hot rolling are restricted, and moreover the microstructure after hot rolling is also restricted.
The Patent Literature 3 discloses “a steel for gears” in which Si: 0.1% or less and P: 0.01% or less and so on are regulated and which provides highly reliable gears having high strength and high toughness.